Automatic switch for telephone systems



Nov. 17, 1953 H. w. BALZER ETAL AUTOMATIC SWITCH FOR TELEPHONE SYSTEMS 3 Sheets-Sheet 2 INVENTORS Harvey W Balzer BY Pier Bakker wfk/ Aflys.

Filed Aug. 25. 1950 m9 xmoziwz 53mm Nov.'l7', 1953 H. w. BALZER ET AL AUTOMATIC SWITCH FOR TELEPHONE SYSTEMS 3 Sheets-Sheet 5 Filed Aug. 25 1950 mmh mmm So NS Atfys.

INVENTORS Harvey W Ba/zer Piqr Bakker w no. mokomzzoo I). III;

Patented Nov. 17, 1953 FEMS AUTOMATIC SWITCH FOR TELEPHONE SYSTEMS Harvey W. Balzer, Downers Grove, and Pier Bakker, Qh-icago, 11]., assignors to Automatic Electric Laboratories, Inc, Chicago, 111., a corporation of Delaware Application August 25, 1950, Serial No. 181,511

11 Claims.

The presentinvention relates to automatic telephone systemsin general, but is concerned more particularly with improvements in automatic switching apparatus to be utilized in completing connections to called subscriber substations from a dial toll switchboard of the type disclosed in the copending Balzer application, Serial No. 181,- 598, filed August 5, 1950.

In a telephone system provided with a dial toll switchboard of the type disclosed in the above mentioned Balzer application, telephone connections are completed from the switchboard by" means of a key controlled impulse sender. This impulse sender is arranged to transmit switch setting impulses to an automatic switch assoeiated with .a link at the position over the same EC conductor that is utilized to return various supervisory signalsto the toll operator position. When a dial toll switchboard of the type noted is installed in an existing telephone exchange equipped with automatic switching apparatus of the type which is controllable by loop type switch setting impulses and Which-is also arranged to return loop type supervisory signals, it 'is norreally necessary to remove such switching app-aratus and replace the same withE'C type impulsin and supervisory switching apparatus.

Accordingly, the principal object of the present in ention is to provide in a telephone systern of the type noted arelay network arranged to permit EC type imp-ulsing and supervisory switching apparatus tobe utilized in an existing telephone exchange equipped with loop type impulsing supervisory switching apparatus in order to avoid the necessity of replacing such existing equipment.

It is another object of the invention to provide in a telephone system of the type: noted, a re lay network which is adapted to receive EC type impulses from, the impulse sender at the dial, toll switchboard and to. convert such impulses into loop type ilnpulses in order to control the setting up of. connections.

It still another object of theinvention to provide in a relay network of the type noted above, facilities for converting loop type supervisory signals returned thereto into EC super viscry si'mals inorder to transmit such signals to the toll operator position.

It isa further object of the invention to provi e in a telephone systeman improved automatic switch which is arranged to receive and transmit control signals over the EC conductor of an incoming trunk line and which is also arranged to receive and transmit control signals over the line conductors of an outgoing trunk line.

I It is a still'further object of the invention to provide in a telephone system an improved automatic switch which is arranged to be controlled by EC impulsing, which is arranged to control subsequent automatic switches of the loop impulsing type and which is arranged to return loop type supervisory controls received thereat over the EC impulsing conductor.

Further features of the invention pertain to the particular arrangement of the circuit ele ments of the relay network and the automatic switches whereby the above-outlined and additional operating features thereof are attained.

The invention, both as to its organization and method-or" operation, together with further objects and advantages thereof, will best be understood byreference to the following specification taken in connection with the accompanying drawings, in which Figure 1 is a schematic trunkmg diagram illustrating portions of a switch train embodying the present invention; Fig. 2 schematically illustrates an operator position at a dial toll switchboard, a link thereat and an individually associated toll preselector over which connections may be established by the toll operator to an idle one of a plurality of selectorsoneof which is disclosed in Fig. 3; Fig. 3 shows the'details of a relay network and such portions of a selector, individually associated therewith, as are deemed necessary for an understanding of the invention; and Fig. 4 shows the details of a relay network and such portions of a connector, individually associated therewith; as are deemed necessary for an understandin of the invention. Fig. 4 also shows such portions another type of a connector as are deemed necessary for an understanding of the invention and a plurality of a called subscriber substations accessible to the illustrated connectors.

Referring now more particularly to Fig. 1, it should be noted that the operator position 22cc andthe. impulse-sender thereat (not shown), the

link. E3526 and the toll preselector Milt are the same as the corresponding apparatus described and illustrated in the above mentioned Balzer application.v The selector use is of the wellknown Strowger type which is selectively operable under control'of the impulse sender to raise its wiperset step-by-step in a vertical direction in accordance with the impulses transmitted thereto and which is thereafter automatically operable to rotate its wiper set to find and select an idle trunk line terminating a relay network, such as I 08, and an associated selector, such as 500. The selector I09 is also of the wellknown Strowger type having two wiper sets which are selectively operable step-by-step in a vertical direction in accordance with impulses transmitted thereto and which are automatically operable in a rotary direction to find and select an idle trunk line terminating a relay network,

such as IO0A, and an associated connector, such as I05, if one of the wiper sets is effective or to find and select an idle trunk line terminating a connector, such as I05, if the other wiper set is effective. The selector I00, for example, may

be of the type disclosed in Fig. 1 of the copending Bakker application Serial No. 181,510, filed August 25, 1950. The connectors I05 and I05 are also of the Well-known Strowger type having wiper sets which are respectively selectively operable step-by-step in both a vertical and a rotary direction to select particular called subscriber lines. These connectors may, for example, be of the type disclosed in Fig. 60 of the text book entitled Automatic Telephony, sec 0nd edition, written by Arthur Bessey Smith and Wilson Lee Campbell and published in 1921 by the McGraw-Hill Book Company Inc., New York, N. Y. It should be noted, however, that the connector I05 has been modified to selectively connect a high resistance to the line conductors for the purpose of returning supervisory controls to the calling toll operator instead of returning re verse battery supervisory controls over the line conductors in accordance with conventional operation as is disclosed in the connector I06.

In order more clearly to set forth the novel features of the invention disclosed in Figs. 2, 3 and 4, it will first be assumed that the operator attending the operator position 2200 desires to extend a connection to the subscriber at substation T. It will also be assumed that the operator has selected the link I300 and has registered appropriate digits in the impulse sender at her position for routing the connection via the toll preselector I400, the relay network I08, the selector I09, the relay network I04A and the connector I05 to the called subscriber line. As is described in detail in the above mentioned Balzer application, the impulse sender at the operator position transmits impulses corresponding to the first registered digit over the EC conductor CI003 to the pulse relay RI420 in the toll preselector I 000 and it applies ground potential to the C conductor CI IM in order to hold the selector I400 from the operator position. The pulse relay RI 020 in response to the impulses of the first digit transmitted thereto controls the vertical magnet (not shown) to raise the wiper set prising the and line wipers I493 an'l I090, the EC wiper I495 and the C Wiper I091 step-by-step in a vertical direction to a level in the associated bank contact set corresponding to the number of impulses of the first digit. Thereafter, the rotary ma net (not shown) of the selector I400 is automatically controlled to rotate the Wiper set over the individual contacts in the selected level to search for and seize an idle trunk line extending to a relay network and an associated selector, such as the relay network I 00 and the selector I09.

The relay network I08, which is individually associated with the selector I09, is marked as idle to the selector I400 by an absence of ground potential and a presence of battery potential, through the winding of a control relay R35 line relay R300 now operates on the C conductor C305. It will be assumed that the wiper set of the selector I 400 seizes the idle relay network I03 over the trunk line comprisinng the EC and C conductors C302 to C305, inclusive. When this occurs, the switching relay RI450 is operated in the selector I000 thereby to connect the and EC conductors CIIIOI, CI402 and CI003 via the Wipers I093, I494 and I005 to the conductors C302, C303 and 0300. Also, the relay RI460 applies a holding ground potential via the wiper I03? to the C conductor C305 and it disconnects the pulse relay RIfiZi from the EC conductor CI003. The pulse relay RI 420 in the selector I 400 now restores to normal and the switching relay RI050 is held in its operated position by the ground potential applied to the C conductor CIGM from the calling operator position.

Referring now to the relay network I00, it Will be seen that the ground potential applied to the C conductor C305 of the incoming trunk line completes a circuit, including the contacts 353, for operating the control relay R360. As soon as the relay R350 operates it looks itself via its contacts 30I to the grounded conductor C305. As a further result of the operation of relay R360, at its contacts 365, it applies ground potential to the C conductor C350 extending to the selector I09. As a further result of the seizure of the relay network I08, a circuit is completed for operating the line relay R340 via its low resistance lower Winding. More specifically, as soon as the toll preselector I400 seizes the relay network I08 and connects the EC conductor CI003 to the EC conductor C300 the battery potential applied thereto by the impulse sender at the operator position 2200 is extended via the resistance lower winding of the line relay R300 to ground at contacts 35L Consequently, the and completes an obvious circuit for operating the slow-to-release hold relay R330. As soon as the hold relay R330 operates, it completes, at its contacts 332, an obvious circuit for energizing the right hand winding of the shunt field answer relay R3I0 but this relay will not operate its contacts 3H and 352 until the current flow through the circuit for energizing the left hand winding is in the proper direction. As a further result of the operation of the hold relay R330, it completes a loop circuit, at its contacts 33I, for operating the line relay R380 in the selector I00. The last mentioned circuit may be traced from ground potential in the selector I09 via the lower winding of the line relay R380, the contacts 313, the line conductor C355, the contacts 34I, the lower right hand winding of the repeating coil 30L the left hand winding of the answer relay R3I0, the upper right hand winding of the repeating coil 30L the contacts 33L the line conductor C355, the contacts 37! and the upper winding of the line relay R380, to battery. It should be noted that the above traced loop circuit completes an energizing circuit for the left hand winding of the answer relay R3I0 and the upper and lower windings of the line relay R330. The line relay R380 operates, but the current flow through the left hand winding of the answer relay R3I0 is in such a direction that it opposes the energizing circuit of the right hand winding of the relay and prevents it from operating. In response to the operation of the line relay R300, an obvious circuit is completed for operating the hold relay R305 and the latter relay, at its contacts 386, prepares a point in the eye-semi .ci'rcuit cfor transmitting witch-*s'ettingiiifipulses al ffur ther result of the peration' 'olrelay-' RZQE, at dts conta'cts iiiih l'fl applies -a*:mii1tip'-le holding ground potential to thei conductor *(3355.

r -The relaynetwork itt' andthe'seleotor i iii are now' in "condition to respondto the second digit transmitted b "the impuise' sender at the opera torlposition-si tt. --l';onsequentl y, the second i'git transmitted by the impulse sender 1 interrupts the circuit for :the line relay R349 which may re stores and reoperates the 'samenu'mbe'r o'f times as' the number oi'inipu'lses in thedigit transmitted. Upon the firstkrestoration df theline relay R358, it completes a circuit via the-contacts' 342 and 333 for operating the .dial'i-'el-ay"R;ii it and, at contacts 3 5i, it if 'rrupts point 'in'-thepreviously traced loop circuit for-controlling th'e 'line relay Rtllfi. As soon a the-dial rel-ay fi lt toperates, it completes, at its contacts 322, a circuit for removing the impedance of the upper right hand winding of the repeating coil "5159i 'a'ndthe left'hand winding of the answer relay iiiirom the previously traced loop circuit. The dial R320 is of the slow-to-release type andremains in its operated positiontlui-ing the impixlsingoperation" of the line relay nets. 'Itshoiild also be noted at this time that the hold relay RSS9 is also of the 'slow to-release typeenii consequently, remains in its operated position duringthe'iinpulsingoperation o'f'the line'relay R356 'Each'-'time the"loop circuit for theline relay Rttil is interr upted at the contacts 3'41, the relay restores to normal and transmits, at 'itscontacts 382, impulses to the vertical magnet (not shown) 5 of the selector "lot. Each time the contacts 3 are closed, the line relay R385 reoperates an'din'terrupts, at its contacts 35:2, the-circuit for thea ertical magnet. The hold relay Rt'fic is of the slow-tor-elea-se type and consequently, remains in its operated position during the impiilsing of the line relay R389. In'this manner, the "impulses received over the EC conductor C-3tleontrcl the "line relay new and the latter-relay, at

its contacts 345, controls the linerlay-"Rtiiii to control the vertical magnetof the selector his to raise the wiper sets step-oy -step in a vertical direction. Thetwo wiper sets comprising the and C wipers liii, iilz and 583 andthe net (not shown) of the selector lllfi isaotomatic- 5:,

ally operated in the manner described in the above mentioned Bakker app'lication 'to step the wiper sets in-a rotary direction "to Search for-an idle trunk line. In the present example; it Will be assumed that the wiper switching relay'RSlli! remains in its normal unoper'ated position in orrler to render the first wiper set 'comprising' 'the wipers as! 'to 393', inclusive-effectiveduring the rotary stepping operation.

Attention is directed to'the'iact that the'rotary stepping of the wipers ofthe selector its takes place at the end of the series of impulses or the second digit of the called numbertransmitted to the relay network 198. At theend of the last impulse of the series, the line relayRtMJ remains in its operated positionin order'tomaint'ain the loop circuit for the line relay R38ll, at its contacts 3M, and in order to retain the slow-to-release hold relay R3313 in its operated position. However, at its contacts 342, the relay-R342 inand C wipers lii i, i-Qii and. 6% are raised relay :now cslowlytrestores to :normal. I the zrelayl tfiiu restores to normal, it'interrupts,

at iits icnn'tacts 32 2, the previously 1 described circilit .short circuiting the xnpper right hand winding of theirepeating .coil "3t! and the left *hand winding :of the. answertrelay. R3 lfi. Also, at its contacts 321?, the relayiRtZG connects "thefline termination network comprising the condenser liz land-ltherresistance '323 across-the upper and lower right hand'windings of the repeating coil 3M :incorderto dissipate anyvoltage change that may have :been accumulated :in the windings of the repeatmgvcoil :Eiiii and the condenser therebetween.

Beferrin'gnow to'the'relay network iiltA I(Fig. 4 gcit will :be' seen that .the incoming trunk line conipiisessthe andxC conductors 0495,..0492 andcGiQ-S. 'Theconduc'tor C t-93 is marked with a ground "potential in the event that the relay network EMA san'd the connector i535 are busy anditheiconductor isfmarked by an'absence of potential when th'ezrelay network will and.the'connector Hi5 are idle. A busy marking ground'potential $011 the 1C conductors of 'busy'trunk lines encountered by the'wiper. 493 during the rotary stepping 10f the selector I09 completes a circuit .via the contacts 13% and Eli for automatically stepping the wiper sets to the next' set of contacts in'ithe selected level. When an idle trunk line is encountered; such as, for example, the trunk line "extending :to therelay network EMA, theiautomat'ic stepping of the wiper sets of the selector I I BB. is terminated and the switchthrough relay RIM-(i is "operated. At this'time, the wipers all! sea and. 453 are in engagement with the condoctors Ciel, C492 and C ltt respectively and the 'switchthroughrrelay Rii'iiiis in its operated position. -As :soon as the .switchthrough relay Biilfi operates, it disconnects, at its contacts 3?! and 3Y3, the loop circuit in the relay network tilt, including the conductors C355 and C356. from fthe linerelay'Rssfl and, at its contacts 312 and '3 mt connects the loop circuit via the con tacts cs1 and 3-93 and thew'ipers till and 1332 to the conductors 6% and CtQZ. Asa further result :o'f-the operation of the switchthrough relay'R'Zi EG', atits contacts 315,:it'interrupts the circuit forthe hold relay R385 which now restores to normal" andfa't its contacts 381, disconnects one of the multiple "holding ground potentials from the C conductor C358. Finally, at its contactsSiS, the switchthrough relay Rtlfl connects the ground potential, at contacts 385, via the C conductcrCE-it, contacts 3'55 and and the C wiper 4113 to (the *C conductor C593.

In view of the foregoing, it will be understood that wh'en the selector Hi9 seizes an idle trunk line extending toa relay network, such as was, and switches thrcugh,the loop circuit of the relay network i'llais connected to the seized idle trunk l'ine and theholding groun' l'potent al on the- C oonductorC zttis foiwva'rtied to the C com doctor of the seized idle trunk line. It should be -noted; however, that the ground potential an pliezil'to the Gconduc-tor C353 also holds the lector' its in its operated position by retaining the switch hrough re'layliltlii operated.

Referring now to the connector i535, illustrated in Fig. 4,.i't willahe seen that'wh'en the loop oicuit 0f the relay network is connected to the incoming trunk 'lineconductors Still! and G532 a circuit is completed for operating the marginal relay R 525 in thecrelay network l iltA in series withthe upper and slower "windings .of the line 7 relay R450 in the connector I05. The last mentioned circuit may be traced from battery via the upper winding of the line relay Refill, the conductor CQS'I, the winding of the marginal relay R4328 (connected in multiple with the resistor 123 and the electrolytic condenser 22), the contacts 4H, the conductor C495, the wiper dill, the contacts 39E and 312, the conductor C355, the contacts 33!, the upper right hand winding of the repeating coil Sill, the left hand winding of the answer relay Rtlii, the lower right hand winding of the repeating coil 39!, the contacts 35!, the conductor C355, the contacts 3M and 393, the wiper 382, the conductor C592, the contacts the conductor 0 398, the lower winding of the line relay R652 and the contacts 583, to ground. Ihe line relay RAM in the connector ldii operates over the above traced loop circuit and, at its contacts 455, completes an obvious circuit for operating the hold relay R 355. w

Referring to the relay network 3A it should 1 be noted that the marginal relay Size is also operated in series with the line relay B l-iii over the above described loop circuit and, at its contacts 22i, completes a circuit via the contacts M5 for operating the digit absorbing relay R 538. As soon as the relay R438 operates, it looks itself via its contacts 232 to ground at contacts 121 and, at its contacts it connects the control relay R-G lt to the grounded C conductor 0 3%. Finally,

at its contacts @535, the relay R 338 interrupts a point in the circuit for the battery reversing relay R iiil. The relay Etta operates as a result of its connection to the grounded C conductor G293 and, at its contacts 33, looks itself to the last mentioned conductor over a circuit which is independent of the contacts 233. Finally, at its contacts 365, the relay E i-it interrupts a point in the initial operating circuit for the relay Rees. With the relays B 223, 343$ and R i lfl in the relay network .iiitA in their operated position as noted above, the marginal relay R 325; may respond to subsequent impulses transmitted over the loop circuit to the line relay R 558 but due to the slow-to-release characteristics of the digit absorbing relay Rdllil, the latter relay remains in its operated position during any impulsing oper ation of the relay R525.

The last two digits of the called subscriber number are transmitted over the EC conductor C3154 in order to control the line relay R3 38 and the last mentioned relay, at its contacts 3- l controls the above described loop circuit in order to repeat the last two digits to the line relay E 553 and the marginal relay R625}. ihe line relay acts in the connector 3&5 restores and reoperates in the usual manner under control of the impulses of the next to the last digit and, at its contacts @522, repeats the impulses to the vertical magnet of the connector The vertical magnet controls the vertical stepping of the wipers i-85 to inclusive, to a level in the associated bank contacts terminating the particular called subscriber line conductors 0238 to G ide, inclusive. During the intcrdigital pause between the next to the last and the last digit, the connector auto matically functions in the well known manner to prepare the circuit for the rotary magnet (not shown) so that the last mentioned magnet will respond, under control of the line relay Refit, to the last digit of the called subscriber number. Consequently, the rotary magnet steps the wipers 485 to 81, inclusive, over the contacts in the selected level to engage the particular contacts terminating the called subscriber line. When this occurs, assuming that the called line is idle, the switchthrough relay RAW in the connector m5 operates and at its contacts 63! and 432 it causes the transmission of ringing current over the called subscriber line to signal the called subscriber substation. Also at its contacts 48 3, the relay R486 connects ground potential via the wiper 48'! to the C conductor C ifili in order to control the line circuit of the called line in a conventional manner and in order to mark the called line busy.

As previously noted, the marginal relay R429 may respond to the impulses 0f the next to the last and the last digits transmitted to the connector 5% but due to the slow-to-release charcteristics of the relay R430, the last mentioned relay remains in its operated position. However, as an added precaution, the electrolytic condenser s22 is bridged across the winding of ie relay R620 in order to prevent the last mentioned relay from restoring to normal long enough to inadvertently restore the relay Refill when the relay network EMA is connected to a high impedance line. In the present example, the relay network i MA is connected in a low impedance line circuit and consequently, the condenser 422 has little or no effect upon the operating characteristics of the relay R ZZQ.

As previously noted, the switchthrough relay R lSii in the connector its causes the transmission of ringing current to the called subscriber substation. In addition to the foregoing, the relay R4663, at its contacts 383, removes the direct ground potential from the lower winding of the line relay Rfific and thereby removes the short-circuit from the high resistance 46!]. As a result of the last mentioned operation, the high resistance 68% is now included in the previously traced loop circuit including the windings of the line relay R 358, the marginal relay R428 and the left hand winding of the answer relay R310. The value of the resistance E59 is sufficient to cause the marginal relay R526 to restore, but it is insufficient to restore the line relay R lfifi. In response to the restoration of the marginal relay Eda-El, the circuit is interrupted at the contacts 32! for the digit absorbing relay R636. The latter relay slowly restores to normal and, at its contacts :33 l, prepares a point in the circuit for the battery reversing relay RAW.

When the called subscriber at substation T answers the call, a ring-cut-off relay (not shown) in the connector N35 operates in order to disconnect the ringing current from the called subscriber line and in order to connect the upperand lower windings of the back bridge relay R l'ifi via the contacts 381 and 482 and the wipers 585 and 386 to the and conductors 03% and 0485) of the called subscriber line. As a result of the foregoing, the back bridge relay R ll'il now operates over the loop circuit including the called subscriber line conductors C483 and (M89 and the called subscriber substation instrument. As soon as the relay R410 operates, it completes, at its contacts All, a circuit for again applying a direct ground potential to the lower winding of the line 9 relay R456 in order thereby teshort circuit the high. resistance 569. Since the high resistance 469 is now removed from the previously traced loop 'circuitfifor the windings or the relay R656 and the marginal relayRE't, the last mentioned relay reoperates. this time, the reoperation of the marginalrelay completes a circuitvia the contacts 52i, set and it! for operating the battery reversing relay new. At its contacts 4| 1 to em, inclusive, the relay R iiiinow reverses thec'onnection's between the line contiu'ct'o'rs Qt iil and csez and the line conductorsciisland Cite. This reversal in the connection or theahove n'o'tei line conductors reverses the direction of the current flow through the Ieithand vinr'iing of the answer relay new in the relaynetwork its; In other words, the left hancl winding of the answer relay R3 l e i'snow energised'in thesame oil-action as th'e energizing circuit forthe right 'wi 21- mg ofthe relay and. thereby causes the relay to operate itscontacts 3 1! and M2. The contacts 3! i of the relay Riiii removestheline' termination network including the condenser 32 i and theresistor 323 from across the line conductors C355a'an'd C35 3 and the contacts 312 complete an obvious circuit for operating the supervisory relay R359. As soon as the relay Refill operates, it removes ground potential, atits contaetssehfrorn thelow resistance lowerwinding of. the line relay 334s. and simultaneously therewith, at its contacts 852, it connects ground potential via the high'resista'nc'e upperwindi'ng' and the low resi'stance lower winding of the line relay Rtehin series, to the EC conouctor'CMi l. The substitution of the high resistance for thelo vv resistance returned over the EC conductor C3631} to the o erator position 22% gives the operator thereat an oiiho'ok answer supervisory signalv In passin'g,v it should be noted that the impulse sender at the operator position, having completed the transmission of the called subsoribernuinber over the E conductor Ci ies, has tlisconnectetl itself from the last mentioned conductor and has substitutecl a su ervisory relay at the operator position. Upon receipt of the answering supervisory signal, the operator at position 22 to may converse with the called subscriber at substation 'I over thecstablisheti connection including the line'c'onductors Cited anal cases of the toll preselector one, line conductors C382 and" C383, the inductively coupled windings oi" the repeating coil Bill, the line conductors C355 and @356) the line conductors C iti and 0 3%, the line conclude tors Owl and Ctfi aand the line conductors QCQBB and C53 9 of the called subscriber line. It shoul'cl bBlnOted, hfifi Vl, that the resistor 323' is'bri'clgecl around the winding of the relay R425} in order to provide a talking path which is independent of the winding of the relay nest;

The established connection is held under controlof the operator at the operator position 22%, by battery potential forwarded over the conductor Ctiilsi to the line relay Elite andby g'rounri' potential forwarded over the C conductor Citll l It will be recalled,-

to the toll preselector i l ii. however, that the preselector was, in" turn; forwaiiclsa' holding ground potential over the CS cons ductor 03th; to the control relay Rttil,

Before ole-scribing the release of the estab lished connection, it will he assumed that the called subscriber at substation 1 momentarily actuates the switehhool: in order to signal the calling operator. the switchhook 1s momentarily depressed, the back briclgei relay: Rl 'iil-isxmomentarily restoreciatoinormal and, at

its contacts Ml, momentarily inserts the high resistance 468 in the previously described loop circuit including the relays R428 and R 559. The relay Refill, as previously noted, restores to normal each time the resistance @69 is'in'clud'ed in the loop circuit antlit reope'rates each time the resistance see is short-circuite'zi. Consequently, at its contacts $25,- the relay Refit restores and reope'r'ates the battery reversing relay acre and the latter relay, at its contacts ii l to lls, inclu si've, alternatelycontrols therclir'ection of current flow over the line conductors from the reverse direction to the normal direction thereby to cause the answer relay to momentarily restore and reoperate. As a result of the foregoing, the answer relay, at its contacts 3H3; momentarily interrupts the circuit for the supv'ervisory relay the latter relay, at its contacts 325i alternately returns low and high resistance ground potential over the EC conductor Ctiifii in order to transmit the switchho'ok supervisory, signals to the calling operator at position 22%.

It will now be assumed that the called subscriber at substation T releases the connection by hanging up the receiver on the associated switchho'ok. As a result of the last mentioned operation, theback bridge relay new in the connector Hi5 restores to normal and, at its contacts ill, inserts the resistance the in. the previously traced loop; circuit. Consequently, the relay R529 restores to normal and, at its contactsefil, interrupts the circuit for battery reversing relay R-tisl. The relay new, at its contacts Mi and. 4M, inclusive, rees'tablishes the normal cur-- rent flow over the loop circuit and thereby causes the answer relay Riilll in the re ay network [38 to restore to normal. The last mentioned relay, at its contacts 312, interrupts the circuit for'th'e supervisory relay R353 which relay now restores and, at its contacts 35!, substitutes the low resistance; grounch including the lower winding. of

theqrel'ay nste; for the high resistance ground,

including, the" highresistance: upper winding and the low resistanceilowerf winding oi the line re= lay onv theEC conductor: case; This sub stitutionreturns at steady lli'JW' resistance on hoolr"supervisory signalito: the operatoriposi tion, 22 iniorder to inflicat'e to the operator.

L thereat that: theme-tilled: subscriber has releaseol The toll operator at position z llo may now release the'iestahlishericonnection-by actuating a release key associatedwith' the link. i 3%: thereby toyre'move-the-batterypotential;from the EC con-' ductor Cilflotandinorder'to remove the ground potential ir'o'ma the Gconcluctor onset.

suit of the: removal-toi ground potential fror'n 'the C conductor Cilt hthe switching relay esteem the; toll preselecton iilfierestores tonorh'ialthere by to: remove'uthe direct ground potential from the C conductor 03% extending 'tothe relay not work PM; As afurtherl result of the restoration of the switchingrelay, Ri ist the release magnet (not shown) in the'tolrpreselector Mitt}: istactu ateti thereby to-causethe wiperset including the wipers 549% to M; inclusive, and i iQ'l-Qto res store to their normal resting positions and rem tier the: toll preselector l ltils available or aclel-i tional calls-in the manner describejcl in; the pre vi'ously mentionel-Ealzer application.

Referring now to-the relay network i as, ,itwill;

be seen that-when; theabattery potential: isre: moved from the conductor C3915; thealin'e" ro lay nseethereat-restores; to normal anmat its: contactsrt iii, interrupts thelcoo circuit for the ii line relay in the connector As a further result or" the restoration of the line relay R359, at its contacts it interrupts the circuit for the slow-to-release hold relay R335- which also restores to normal. During the restoration period of the hold relay a circuit is completed for the l relay R321! but operation of the relay R325 at this time of no importance. As soon as the ground potential is removed from the control conductor 03%, the control relay R358 in the relay network i533 restores to normal and connects the winding of the supervisory relay R553 via the contacts 38 to the C conductor C35 extending to the selector it. At this time, the supervisory relay may be operated from ground potential at the contacts 456 in the connector returned over the C conductor C 553. In other words, the supervisory relay R358 will retained in its operated position until the connector m5 has fully restored to normal. As previously noted, the relay R358, in its operated position, returns a high resistance ground potential to the EC conductor 03% but in addition thereto, at its contacts 35 3,

it now connects ground, at contacts 352, to the C conductor C335. The application or ground potential to the C conductor CW5, at the present time, maintains a busy marking ground potential on this conductor in order to prevent another toll preselector, such as Mild, from sei ing a relay network 538 until the switching apparatus employed in the present connection has been restored to normal. In addition to the foregoing, the ground potential returned over the conductor 0593 via the wiper and the contacts 3% and 3115 to the C conductor C358 retains the switchthrough relay Rii'ifii in the selector its in its operated position. Therefore, the selector 5% cannot restore its wiper sets to their normal positions until the switchthrough relay R375 is restored to normal.

Referring now to the connector I05, it will be recalled that the line relay R558 was retained in its operated position over the previously traced 100p circuit including the relay network 558. Consequently, when the line relay R349 restores to normal, as previously noted, it interrupts, at its contacts 3M, a point in the above mentioned loop circuit and thereby causes the line relay R450 to restore to normal. Upon restoring, the relay R555 interrupts, at its contacts iiii, the circuit for the slow-to-release hold relay R455. The latter relay, upon restoring to normal, interrupts a point in the circuit for the switchthrough relay R439 which also restores to normal. In addition to the foregoing, the hold relay R455 completes a circuit (not shown) including the contacts 652 for operating the release magnet of the connector I05, which magnet causes the wipers 485 to 48?, inclusive, to restore to their normal positions. Incident to the restoration of the relay R455, at its contacts 456, it removes the holding ground potential from the C conductor 393 and thereby causes the control relay R 38 in the relay network lEl lA, the switchthrough relay R379 in the selector I09 and the supervisory relay Rfiiifi in the relay network I08 to restore to normal. The restoration of the control relay R445 prepares, at its contacts Mi, a point in the circuit for controlling the digit absorbing relay R435. Consequently, the relay network 104A and the connector IE5 are now in condition to be utilized in another call. In the selector I09 the restoration of the switchthrou h relay R319 completes a circuit, at its contacts 315, for operating the release magnet (not shown) in order to cause the wiper sets includ ing the wipers ll to 406, inclusive, to restore to their normal resting positions. In addition to the foregoing, the switchthrough relay Rai at its contacts 3?! and 313, reconnects the windings of the line relay R389 to the line conductors C555 and C356. Consequently, the selector lilii is fully restored to normal and is rendered available for an additional call. In the associated relay network l08 the restoration of the supervisory relay R359 removes the busy marking ground potential from the C conductor C395, at its contacts it connects the low resistance lower winding of the line relay R349 to the EC conductor 0354!, at its contacts 35!; and, at its contacts 353, it recon nects the control relay R359 to the C conductor C305. All of the switching apparatus involved in the established connection between the calling operator at position 2265 and the called subscriber substation T are now restored to normal and rendered available for additional calls.

In the previous description of the operation of the relay network Hi8 and the selector I89 under control of the impulse sender at the operator position 2200, it was assumed that the selector i3 9 seized the relay network ld lA over the moors trunk conductors C45! to C 393, inclusive, tilizing the first wiper set including the wipers 401 to 453, inclusive. It will now be assumed that the wiper switching relay R390 is in its operated position at the time the selector H39 rotates its wipers over a selected level. Consequently, the second wiper set including the wipers $64 to 356, inclusive, will be rendered effective to search for an idle trunk line terminating a connector, such as the connector H16. With the wiper switching relay R390 in its operated posi tion, the contacts 392, 394 and 395 are closed and the latter contact completes a testing circuit for controlling the rotary stepping of the wipers until an idle trunk line is seized. For the purpose of this description, it will be assumed that the connector I06 is idle and therefore no busy marking ground potential will be applied to the incoming trunk line C conductor C496. Consequently, when the selector Hi9 engages the idle connector 35, the switchthrough relay RBIs will be operated, in the manner previously described, in order to extend the operating loop circuit from the relay network N18 to the line relay R529 in the connector I06. This loop circuit may be traced from battery via the upper winding of the relay R526, the contacts 5! l, the conductor C494, the wiper MM, the contacts 392 and 3'52, the conductor C355, the contacts 33!, the upper right hand winding of the repeating coil Sci, the left hand winding of the answer relay the lower right hand winding of the repeating coil 30!, the contacts til, the conductor C35 5, the contacts 374 and 394, the wiper 405, the conductor C495, the contacts 513 and the lower winding of the line relay R528, to ground. The line relay R525 operates over the above traced loop circuit and in response to the last two di ts of the called subscriber number selectively controls the vertical and rotary magnets (not shown) to position the wipers 53! to 533, inclusive, into engagement with the associated contact set terminating the called subscriber line. The connector thereafter tests the called line to determine whether it is idle or busy and, if idle, trans mits the ringing current to signal the subscri er at substation Tl. When the called subscriber answers the connection, a loop circuit including aesa.

the; line. conductors and; 6.5.3 is: completed for. operating. the bachbridgerelar rtifigtflrinthe; connector lfliil; Upononeratineithehackbrid rel y; R1511}, at its: contacts, 511 to 5111,. inclusive, reverses-the current flow over-the lin condu tors ores and .Ce-iifigthereby toi cause; thaoperationof the. answer relay one; inzthel mariner: previ usly: escribed.

In; View of the-prev ua descriptienpq s the op. e ationcf'therelay t-titit i lhe underood ha sthe op ra io theanswer re ayfitim he. one .rv rrauses he f: "hook".

anernss on of: a; hi h; r si ts. w r pervi orysn l be eueto 'Cte l to he calli on s opera-toratn sit on Bertrand c ll d-sure r jeret ubetation. T1; In no converse over he a li hedonne tio 1nc1uo'ne;; he nk tree; the; toil. presele to. Meet, t e .ler netw rk its. th sel ctor; i .e-ane; th i-oonneotor 1 Qherroma n ne o r ionsr fhe switchi c n: narrat inc uded n theaaboveicescr e .c nnec tion whereby switchhook supervision andfdisconj-- nect supervision are transmitted-to the; calling operator are the sameras has-been described except that the back bridge relay Riiliil in; the. connectar. l-iiii: is. directly controlled. .by the called subscriber in order; to control the answer relay Reid in the relay networlg M38. The principal difference between the; connector; lee-and the connector its is; that the former controls the. transmission of supervision byincludingor excluding-a high. resistance. in, the loop circuit. for the line relay whereas the; latter connector controls; the transmission of supervision by m versingthe direction of-current flow overthe loop circuit for theline relay-R526;

The release of the established connection is under control of the calling operator at position 22% and since the releasing operations of the apparatus includedinthe connection to-the calls subscriber atsubstation- Tl are. exactly the same asv described above in releasing a connection tothe; called subscriber at substation T,.they need notbe repeated.

In the setting upof a-connection toeither -thecalled subscriber at substation I or the called subscriber at substation T1, the sel.ector lee-automatically rotated its wiper sets; overthe contactsv of a selected level to. search for an idle trunk line over the'first wiper set including the. wipers till to rilw inclusiye or over the. second wiperset including the wipers the; to 4%, inclusive, depending upon whether the wiper switching relay R396 is: in its restored position or in its operated position. In, either event, if

the selector Hill; fails to find an idle-.trunk line in. the selected-level, the wiper sets-will-be rotated the EC conductor C357! and the-contacts to the winding. of the supervisory relay R350.

The supervisory relay R3553, will, operate and res-t store under control of. the groundpulses transmit-ted over the abovetraced. circuit and. alter-w nately open and close. its contacts 3.5.1:. and: 3.52;

A'srpreviouslyp noted, these-contacts arescon i4 nested in the circuits for the; upper and: lower windings of the relay Refit; so that either-a low resistance or a. high resistanceground' potential. is returned over. the EC. conductor Cece to the operator position me. At the present time, due. to the all-trunksebusy:condition encountered by the. selector use, the supervisory reiay R359 opcrates and. restoreeat the razteyof 12.8 times per minute. Consequently, 31111115. contacts and 352, it' will alternately return highand low resistance ground potential over the. conductor care to the impulse; sender in;the operator-pcsition site. Attention is. directed to the fact; that: the impulse sender; is still connected inasmuch. s. it has. ot cornpieterl the transmissionor the last two digits of the called subscriber number. The alternate high and. lowxresistance ground potential returned over the conductorCi-tfie'.

controls the impulse sendertogive. the operator at position 22%- an all-trunhs.busy flashing sig.--

nal to indicate that the desired connection can.- not he established at the presenttiine.

When the calling; operator receives the alltrunks-busy flashing condition, she. releases: the.

partially established connection in the manner previously described, thereby to cause the restoration of the toll preselector. Hits: and the restoration of both the line-relay RMQ: and the control relay Rees. inthe. relaynetwcrk me. As soon as the line relay Rfiiizrestores to normal, it. interrupts, at its. contacts Set, the :previouslytraced loop circuit forythegline. relay which relay now restores to normal and interrupts the circuit for the hold relay R385; The latter relay slowly restores to normal and completes a,ci-rcuitror, operating; tn e .-release magnet (not-shown) of. the selector ine. thereby, to. cause the wiper s ts. there f: t re ore. to; their normal. resting positions. As. soon; as; the wipers-start to restoreto normal, the cam. springs CAglifiSfili are opened thereby to disconnect the i2 iiiPM-conductor C3198: from the EC. conductor $33.51.. In. view of the foregoing, it. will be undersi'cod that. when. an.

all-trunks-busy condition. is, encountered: by the selector ice during: the setting. upzof a, connection, the supervisoryrelay ligieii; is controlled to transmit a distinctive; all- -.trun s.-.=busy. signal to thecallingoperator over "the En conductor (33%;

While one emloodimentor the invention has been disclosed it- Will. be understood that; various modifications may bemade therein which. are:

within the-true: spirit and; scope of the inven.- tion,

What is claimed is:

1.. In a telephone, system, an automatic switch comprising an; incoming trunk line including; a pair of line conductorean'da control conductor and an extra control. conductor, a first linere-- conductor. in order selectivelyto energize said first". winding, or. said? first: and second windings depenchngluponi the. completion: of said first i or said second connections respectively, said first line relay being operated when either said first winding is energized or said first and second windings are energized, a second line relay provided with first and second windings, a loop circuit adapted to be selectively connected to said first and second windings of said second line relay in order selectively to energize said windings in series, means responsive to initial operation of said first line relay in response to initial connection of said'source of second potential to said extra control conductor for connecting said loop circuit to said windings of said second line relay in order to operate said second line relay, a wiper set, a line accessible to said wiper set, means responsive to restoration and reoperation of said first line relay in response to one or more interruptions of said source of second potential connected to said extra control conductor in accordance with the impulses of a first digit for interrupting said loop circuit in order to restore and reoperate said second line relay a corresponding number of times, means responsive to restoration and reoperation of said second line relay for operating said wiper set to select said line, means responsive to selection of said line by said wiper set for connecting said loop circuit to said line via said wiper set, and means included in said loop circuit controlled responsive to an answer control returned from said line to said loop circuit for operating said supervisory relay and controlled responsive to a disconnect control returned from said line to said loop circuit for restoring said supervisory relay.

2. In a telephone system, an automatic switch comprising an incoming trunk line including a pair of line conductors and a control conductor and an extra control conductor, a first line relay provided with a first low resistance winding and a second high resistance winding, a source of first potential, a first connection from said source offirst potential via said first winding to said extra control conductor, a second connection from said source of first potential via said second and first windings in series to said extra control conductor, a supervisory relay having a restored position completing said first connection and interrupting said second connection and having an operated position completing said second connection and interrupting said first connection, a source of second potential adapted to be selectively connected to said extra control conductor in order selectively to energize said first winding or said first and second windings depending upon the completion of said first or said second connections respectively, said first line relay being operated when either said first winding is energized or said first and second windings are energized, a second line relay provided with first and second windings, an impulsing bridge circuit adapted to be selectively connected in series with said first and said second windings or" said second line relay, means controlled responsive to initial operation of said first line relay in response to initial connection of said source of second potential to said extra control conductor for connecting said impulsing bridge to said second line relay in order to operate said second line relay, a wiper set, a line accessible to said wiper set, means responsive to subsequent restoration and reoperation of said first line relay in response to one or more interruptions of said source of second potental connected to said extra control conductor in accordi 6 ance with the impuses of a first digit for interrupting said impulsing bridge in order to restore and reoperate said second line relay a corresponding number of times, means responsive to restoration and reoperation of said second line relay for operating said wiper set to select said line, means controlled in response to said selection of said line by said wiper set for connecting said impulsing bridge to said line via said wiper set, an answer relay included in said impulsing bridge controlled responsive to an answer control returned from said line to said impulsing bridge for operating said supervisory relay to complete said second connection and to interrupt said first connection in order to alter the resistance returned over said extra control conductor from low to high, and said answer relay controlled in response to a disconnect control returned from said line to said impulsing bridge reoperating said supervisory relay to recomplete said first connection and interrupt said second connection in order to alter the resistance returned over said extra control conductor from high to low.

3. In a telephone system, an automatic switch comprising an incoming trunk line including a pair of line conductors and a control conductor and an extra control conductor, a first line relay provided with first and second windings that are respectively of low resistance and high resistance with respect to each other, a source of first potential, a first connection from said source of first potential via said first winding to said extra control conductor, a second connection from said source of first potential via said second and first windings in series to said extra control conductor, a supervisory relay having a restored position completin said first connection and interrupting said second connection and having an operated position completing said second connection and interrupting said first connection, a source of second potential adapted to be selectively connected to said extra control conductor in order selectively to energize said first winding or said first and second windings depending upon the completion of said first or said second connections respectively, said first line relay being operated when either said first winding is energized or said first and second windings are energized, a second line relay provided with first and second windings, an impulsing bridge circuit adapted to be selectively connected in series to said first and second windings of said second line relay, means con trolled responsive to initial operation of said first line relay in response to initial connection of said source of second potential to said extra control conductor for connecting said impulsing bridge circuit to said second line relay in order to operate said second line relay, a wiper set, a group of lines accessible to said wiper set, means responsive to subsequent restoration and operation of said first line relay in response to one or more impulses of said source of second potential connected to said extra control conductor for controlling said impulsing bridge circuit to restore and reoperate said second line relay a corresponding number of times, means responsive to restoration and reoperation of said second line relay for operating said wiper set to select said group of lines and for controlling said wiper set to select'an idle one of said lines in said group, means controlled in response to said selection of said idle line for connecting said impulsing bridge circuit to said selected line via said wiper set, an answer relay included in said impulsing bridge circuitop'erated res onsive to an answer control on said selected line, means controlled in response to said Operation of said answer relay for operating said supervisory relay, and a busy device in said switch operated in response to a busy condition of all of said lines in said group for inter nittently operating and restoring said supervisory relay. I 1,

4. In atelephone system, an automatic switch comprising an incoming trunk line including a pair of line conductors and a control conductor and an extra control conductor, a first line relay provided with first and second windings that are respectively of low resistance and high resistance with respect to each other, a source of first no; tenti'al a' first connection from said source of first potential via said first winding to said extra control conductor, a second connection from said source of first potentirl via said second and first windingsin series to said extra control conductor, a supervisory relay having a restored'position cornp'let'ing said first connection and interrupting said second connection and having an operated position completing said second connection and interrupting said first connection, a source of second potential adapted to be selectively connected to said extra control conductor in order selectively to energize said first winding or said first and second-windings depending upon the completion of said first or said second connections respectively', said first line relay being operated when either said first winding is energized or said first and second windings are energized, a second line rele'y'provided with first and second windingsan impulsing bridge circuit adapted to be selectively linerelay in response to one or more impulses of said source of-second potential connected to said eitra control conductor for controlling said impulsing bridge circuit to restore and reoperate said second line relay a corresponding number of times, means responsive to restoration and reoperation of said second line relay for operating said wiper set to select said group of lines and for controlling said wiper set to select an idle" one of said lines in' said group, means controlled in response to said selection or said idle line for connecting said impulsing bridge circuit to said selected line via said wiper set, an answerrelay included in said impulsing bridge circuit operated responsive to an answer control on said selected line, means controlled in response to said operation of said answer relay for selectively operating said supervisory relay in order to complete said second connection and thereby return a high re-' sistance answer supervisory signal over said extra control conductor, a busy device in said switch operated in response to a busy condition of all of said lines in said group for intermittently operating and restoring said supervisory relay in order intermittently to complete said-second and said first connection and thereby intermittently return a high resistance and low resistance busy supervisory signal over said extra control conduetor. v I

51in a telephone-system, an automatic switch 18 t t c m risin scr amin t s l ne i sis g a pair or line conduct rs and a control conductor and anextra control conductcr; a first line re' lay provided with first and" second wi g that are respectively of low resistance and high; f sistance with respect toeach other, a source of first potential, a first connection from said source of first potential via said first winding to said extra control conductor, a second connection ircinsaidsource of potential via said secs and and first windingis series to said extra sensor conductor; a slipervisory relay having a restored completing said first comes tion and interrup 1g said second connection and having an operated position completing said second connection and interrupting said first connec *n, a source of second potential adapted to be so ctively connected to said extra control conductor in order selectively to energize said first winding or said first and second windings depending upon the completion of said first or said second connections respectively, said first line relay being operated when either said first winding is energized or said first and second windings are energized, a second line relay provided with first and second windings, an impulsing bridge circuit adapted to be selectively connected in series to said first and second windings of second line relay, means controlled responsive to initial operation oi'ysaid first line relay in response to initial connection of said source of second potential to said extra control conductor for connecting said impulsing bridge circuit to said second line rela in order to opcrate said'secondiline relay, a wiper set, a group of lines accessible tosaidwi er set, means responsive to subsequent restoration and o eration of said first line relay in response to one or more impulses of sai source of second potential connectedto said extra control conduit tor' for controlling said impulsing bridge circuit to restore and reoperate s'aid second line relay a corresponding-number of times, means responsive to restoration and'reope'ration of said sec: ond linerel'ay for operating said wiper set to select said group of lines and for controlling sa-id wiper set to select an idle one" of said lines in said group, means controlled'in r'esponse to said selection of said idle linefor connecting said impulsing bridge circuit to saidselected line via saidwiperset, an answerrelay included in said impulsing" bridge circuit operated responsive to an answer control onsaidsleted line, means controlled" response-to said operation of said answer relay for se'lec vely operating said su' pervisory-relay in order to complete said second connection'and thereby returner; steady high re sesame answer supervisory signal ove said extra control conductor, off-normal contacts in said switch operated responsiveto a busy con dition enc'ounter'ed by said wipens'et-on all of said lines "in said group; and means including said operated normal contacts io'rin-termittently cont'ro .ing' said su ervisory relay to ar ternate-ly complete saidand said first and thereby" alternately retdrn a hignresi'stancean a low resistance btisysifp fi visory signal over said extra control ontract-or:

6 In a telephone system; a first ic" switch comprising an incomingtrunk lineirie eluding a air of line conductors ands; control conductor and an extra control conductor, a second' automatic1 switch so prising" an incoming trunk lineuncludinga p'- r of line conductors and a cont-rol conductor; a wiper set' for each of said switches each including a pair of line wipers and a control wiper, a first line relay connected to the extra control conductor of said first switch incoming trunk line selectively controlled in ac-- cordance with switch setting impulses of digits transmitted thereto, a control relay connected to the control conductor of said first switch incoming trunk line and operated thereover, a second line relay connected to the line conductors of said second switch incoming trunk line, an additional line relay in said first switch, means controlled in response to the selective control of said first line relay in accordance with a first digit for selectively controlling said additional line relay, means controlled in response to said selective control of said additional line relay for selectively operating said first svitch wiper set to select said second switch incoming trunk line, means controlled in response to the selective control of said first line relay in accordance with additional digits for selectively controlling said second line relay over the line wipers of said wiper set and the line conductors of said second switch incoming trunl; line, means controlled by said control relay responsive to said selection of said second svitch incoming trunk line for applying a control potential to said control conductor of said second witch incoming trunk line by way of said first switch control wiper, a called line accessible to said second switch wiper set, means controlled in response to said selective control of said second line relay for operating said second switch wiper set to select said called line, means controlled in response to said selection of said called line for connecting a high resistance to one of said line conductors of said second switch incoming trunk line and for removing said high resistance therefrom in response to an answer on said called line, means controlled in response to said connection and subsequent disconnection of said high resistance for transmitting an answer supervisory signal to said first switch over the line wipers of said first switch wiper set, an answer relay in said first switch operated responsive to said answer supervisory signal, and means controlled in response to said operation of said answer relay for returning an answer supervisory signal over the extra control conductor of said first switch incoming trunk line.

7. In a telephone system, a first automatic switch comprising an incoming trunk line including a pair of line conductors and a control conductor and an extra control conductor, a second automatic switch comprising an incoming trunk line including a pair of line conductors and a control conductor, a wiper set for each of said switches each including a pair of line wipers and a control wiper, a first line relay connected to the extra control conductor of said first switch incoming trunk line selectively controlled in accordance with switch setting impulses of digits transmitted thereto, a control relay connected to the control conductor of said first switch incoming trunk line and operated thereover, a second line relay connected to the line conductors of said second switch incoming trunk line, an additional line relay in said first switch, an impulsing bridge in said first switch, means controlled in response to the selective control of said first line relay in accordance with a first digit for selectively connecting said impulsing bridge to said additional line relay in order selectively to control said additional line relay in accordance with said first digit, means controlled in response to said selective control of said additional line relay for selectively operating said first switch wiper set to select said second switch incoming trunk line, means in said first switch controlled in response to said selection of said second switch incoming trunk line for connecting said second line relay via the line conductors of said second switch in coming trunk line and the line wipers of said first switch wiper set to said impulsing bridge, means controlled by said control relay responsive to said selection of said second switch incoming trunk line for applying a control potential to said control conductor of said second switch incoming trunk line by way of said first switch control wiper, means controlled in response to the selective control of said first line relay in accordance with additional digits for selectively controlling said impulsing bridge to repeat said additional digits to said second line relay, a called line accessible to said second switch wiper set, means controlled by said second line relay in response to said additional digits for selectively operating said second switch wiper set to select said called line, means controlled in response to said selection of said called line for inserting a high resistance in one of said line conductors of said second switch incoming trunk line, means controlled in response to an answer signal on said called line for removing said high resistance from said one line conductor of said second switch incoming trunk line, means connected to one of said line conductors of said second switch incoming trunk line selectively controlled in response to said insertion and subsequent removal of said high resistance for reversing said connection of said second line relay to said second switch incoming line conductors, a relay in said impulsing bridge operated responsive to said reversal, and means controlled in response to said operation of said relay for altering the connection of said first line relay to the extra control conductor of said first switch incoming trunk line in order to transmit an answer supervisory signal over said extra control conductor.

8. In a telephone system, a first automatic switch comprising an incoming trunk line including a pair of line conductors and a control conductor and an extra control conductor, a wiper set for said first switch, an outgoing trunk line including a pair of line conductors and a control conductor accessible to said wiper set, a second automatic switch connected to said outgoing trunk line, a wiper set for said second switch having access to called lines, a control relay connected to the control conductor of said first switch incoming trunk line and operated thereover, means in said first switch selectively controlled in accordance with impulses of a first digit transmitted thereto over the extra control conductor of said incoming trunk line for operating said first switch wiper set to select said outgoing trunk line, means controlled by said control relay responsive to said selection of said outgoing trunk line for applying a control potential to said control conductor of said outgoing trunk line, a line relay in said second switch connected to the lin conductors of said outgoing trunk operated responsive to said selection, means in said first switch selectively controlled in accordance with impulses of subsequent digits transmitted thereto over the xtra control condoctor of said incoming trunk line for selectively operating said line relay over a circuit including the line conductors of said outgoing trunk line, means controlled in accordance with said selecsteam tive operation of said line relay for selectively operating said second switch wiperset to select one of said called lines, means in said secondswitch controlled responsive to said selection of called line for connecting a high resistance to one of the line conductors of said outgoing line controlled by a subsequent answer signal returnedover said called line for disconnecting said high resistance from said one line conductor of said outgoing line, means" connected to the other line conductor of said outgoing trunk line normally operated in series with said liner'elay restored and reoperated responsive to said conneetion and disconnection of said high resistance to said one line conductor of said outgoing line for transmitting an answer supervisory signal to said first switch over the line conductors of said outgoing trnnlr line, and means, in said first switch controlled responsiv 136 said last mentioned signal for returning said answer supervisory signal over said extra control conductor of said incoming trunk line. I

9. In a telephone system, a first automatic switch comprising an incoming trunk line including a pair of line conductors and a control conductor and an extra control conductor, a wiper set for said first switch, an outgoing trunk line including a pair of line conductors and a control conductor accessible to said wiper set, a second automatic switch connected to said outgoing trunk line, a wiper set for said second switch having access to called lines, a control relay connected to the control conductor of said first switch incoming trunk line and operated thereover, means in said first switch selectively controlled in accordance with impulses of a first digit transmitted thereto over the extra control conductor of said incoming trunk line for operating said first switch wiper set to select said outgoing trunl: line, means controlled by said control relay responsive to said selection of said outgoing trunk line for applying a control potential to said control conductor of said outgoing trunk line a line relay in said second switch bridged across the lin conductors of said outgoing trunk line, a marginal relay in said second switch connected in series with one of the line conductors of said outgoing trunk line, means in said first switch selectively controlled in accordance with impulses oi subsequent digits transmitted thereto r over the extra control conductor of said incoming trunk line for selectively operating said line relay and said marginal relay over a circuit including the line conductors of said outgoing line, means in said second switch selectively controlled by said line relay for selectively operating said second switch wiper set to select one of said called lines, means in said second switch selectively controlled by said marginal relay for conditioning said second switch to respond to return an answer condition on said called line, means in said second switch operated responsive to selection of said called line for including a resistance in said circuit of said line relay and said marginal relay, means in said second switch operated responsive to an answer condition on said called line for excluding said resistance from said circuit including said line relay and said marginal relay, said marginal relay restoring and reoperating in response to said inclusion and said exclusion of said resistance in said circuit, means controlled in response to said restoration and said reoperation of said marginal relay for transmitting an answering supervisory control over the line conductors of said outgoing trunk line to said first switch, and means in said first switch controlled responsive to said answering supervisory signal for repeating said signal over the extra control conductor of said incoming trunk linel 10. In a telephone system, a first automatic s-witc'li comprising an incoming trunk line including a pair of line conductors and a control conductor and an extra control conductor, a second automatic switch comprising an incoming trunk line including a pair of line conductors and a control conductor, a wiper set for each of said switch s each' including a pair of lin wipers and a control wiper, a first line relay connected to the extra control conductor of said first switch incoming trunk line selectively controlled in accordance with switch setting impulses of digits t-ransn itted: thereto, a first control relay connected to the control conductor of said first switch incoming trunk line and operate thereover, a second line relay connected to the line conductors of said second switch incoming trunk line, a second control relay connectable to the control conductor of said second switch incoming trunk line, an additional line relay in said first switch, means controlled in response to the selective control of said first lin relay in accordance with a first digit for selectively controlling said additional line relay, means controlled in response to said selective control of said additional line relay for selectively operating said first switch wiper set to select said second switch incoming trunk line, means operated in response to said selection of said second switch incoming trunk line for disconnecting said additional line relay, means controlled in response to operation of said first control relay for connecting a holding control via the control wiper of said first switch wiper set and the control conductor of said second switch incoming trunk line to the control wiper of said second switch wiper set, means controlled in response to the selective control of said first line relay in accordance with additional digits for selectively controlling said second line relay over the line wipers or said wiper set and the line conductors of said second switch incoming trunk line, a marginal relay also controlled over said line conductors of said second switch incoming trunk line, means controlled by said marginal relay for connecting said second control relay to the control conductor of said second switch incoming trunk line in order to operate said second control relay, a called line accessible to said second switch wiper set, means controlled in response to said selective control or" said second line relay for operating said second switch wiper set to select said calling line, means controlled in response to said selection or said cailing line for connecting a high resistance to one of said line conductors of said second switch incoming trunk line and for removing said high resistance therefrom in response to an answer on said called line, said marginal relay restoring to normal in response to the connection of said high resistance to said one line conductor 01 said second switch incoming trunk line and said marginal relay reoperating in response to the removal of said high resistance from said one line conductor of said second switch incoming trunk line, a reversing relay, means controlled in response to said restoration and reoperation of said marginal relay for completing a circuit including contacts of said operated second control relay for operating said reversing relay, means controlled in response to said operation of said reversing rea wiper set, a line, a line relay, means controllable 1 over said line conductors for selectively operating said line relay, a marginal relay connected in series with one of said line conductors selectively operated in series with said line relay, means responsive to said selective operation of said line relay for selectively operating said Wiper set to select said line, a reversing relay, a first relay and a second relay, means responsive to said selective operation of said marginal relay operating said first relay, means controlled by said operation of said first relay for operating said second relay, means controlled by said operation of said second relay for preparing a point in a circuit for said reversing relay, means operated responsive to said selection of said line for connecting a high resistance to one of said line conductors of said incoming trunk line in order to restore said marginal relay and said first relay, means controlled responsive to an answer control returned over said line for disconnecting said high resist ance from said one line conductor of said incoming trunk line inorder to reoperate said marginal relay, and means controlled responsive to said "reoperation of said marginal relay for completing said circuit for operating said reversing relay in order to transmit an answer supervisory signal over the line conductors of said incoming trunk line.

HARVEY W. BALZER.

PIER BAKKER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,291,950 Long Jan. 21, 1919 1,577,083 Sperry Mar. 16, 1926 1,914,357 Brooks June 20, 1933 2,504,708 Long Apr. 18, 1950 2,579,721 Bakker Dec. 25, 1951 

